Apparatus for balancing the outputs of a stereophonic phonograph pickup cartridge



3,088,741 APPARATUS FOR BALANCING THE OUTPUTS OF A STEREOPHONIC May 7, 1963 D. E. JOHNSON PHONOGRAPH PICKUP CARTRIDGE Filed April 13, 1959 IN VENTOR FIG.

FIG. 7

DONALD E. JOHNSON BY W ATTOR EY assam Patented May 7, 1963 ice 3,088,741 APPARATUS FOR BALANCING THE OUTPUTS OF A STEREOIHONIC PHONOGRAPH PICKUP CARTRIDGE Donald E. Johnson, Couneaut, Ohio, assignor to The Astatic Corporation, Conneaut, Ohio, a corporation of Ohio Filed Apr. 13, 1959, Ser. No. 805,8Q6 15 Claims. (Cl. 27423) The present invention relates generally to the art of recording and reproducing sound and more particularly to the provision of apparatus for balancing the outputs of .a stereophonic phonograph pickup cartridge.

As will be understood by those skilled in the art, the most Widely accepted system for recording binaural or sterephonic sound involves the simultaneous recording of two channels of sound or intelligence in a single record groove or trace. The two channels may be cut as a lateral channel and a vertical channel in the single groove or may be cut into the opposite slanting side walls of a ninety degree V-shaped groove or trace and these channels are modulated in directions downwardly at angles of forty-five degrees with respect to the surface of the record disc. This latter method of recording binaural or stereophonic sound is commonly known by the designation 45-45 in the art and is the system which is now enjoying substantial commercial success. The apparatus of the present invention will be described in connection with the 4545 system of recording and reproducing binaural or stereophonic sound.

One of the main reasons that the 45-45 system has been widely accepted is that the two channels of recorded sound or intelligence are substantially balanced with respect to each other. That is, both channels in the walls of the groove or trace are inclined at the same angle with respect to the surface of the record disc, both channels are disposed equally on opposite sides of a neutral vertical center line, both channels may be made to contain substantially equal sound or intelligence over equal frequency ranges and in general all essential parameters are equal whereby the two channels are inherently balanced. Of course, each of the channels may be separately .and individually modulated as is desired. However, if the same sound or intelligence is recorded in both of the channels the modulations or undulations therein will be equal and in this sense the 4545 system is said to be balanced.

In order to realize the inherent efficiencies of the balanced 4545 system it is necessary to provide a stereophonic phonograph pickup cartridge which is capable of converting the balanced mechanical undulations of the two channels into proportionally and equally balanced electrical impulses or outputs. As will be explained below, prior art apparatus is incapable of reproducing the balanced channels of recorded sound or intelligence as balanced electrical impulses or outputs.

In providing a stereophonic phonograpgh pickup cartridge for reproducing 4545 stereophonic or binaural record discs it is, according to prior art teachings, customary to employ two separate transducer elements and to connect them to a single operatively disposed groove engaging tip by coupling means. Various elastomeric clamping and damping pads are employed to support the transducer elements and the inherent characteristics of each of these pads, its placement, the cement holding it and the degree of compression applied thereto may materially effect the response of the transducer element as- .sociated therewith. Also, the transducer elements vary slightly with respect to their comparative transducing efiiciencies. The coupling means may not be equally compliant to signals from the off-side channels .and may not be equally transmissive to signals from the proper channels. The component parts of a stereophonic phonograph pickup cartridges, including the groove engaging tips, are quite small and may very easily be misaligned when the pickup cartridge is being assembled. All of the above variables effect the balance between the outputs of a stereophonic phonograph pickup cartridge. It is a monumental and almost impossible task to adjust these variables with an absolute degree of accuracy. Thus, reproducing systems of the prior art have been inherently unbalanced and it has been next to impossible to obtain balanced outputs from the stereophonic phonograph pickup cartridge.

It is the prime or ultimate object of the present invention to provide apparatus for .adjustably balancing the outputs of a stereophonic phnograph pickup cartridge.

Another object of the invention is to provide apparatus of the character above described which may be employed with an originally non-adjustable sterephonic phonograph pickup cartridge to balance the outputs thereof.

A further object of the invention is the provision of apparatus for balancing the outputs of existing stereophonic phonograph pickup cartridges with a minimum of modification of the recording system and with a minimum of expense.

A still further object of the invention is to provide apparatus of the character described which is of highly improved and highly simplified construction and which is characterized by its low cost and ease of use.

The above, as well as other objects and advantages of the invention, will become more readily apparent from consideration of the following specification and accompanying drawing wherein there are disclosed certain representative embodiments constructed in accordance with the teachings of the invention.

In the drawing:

FIGURE 1 is a fragmentary side elevational view, partially in section, of the front end portion of a phonograph tone arm assembly embodying apparatus for balancing the outputs of a stereophonic phonograph pickup cartridge associated therewith;

FIGURE 2 is an underside plan elevation of the apparatus shown in FIGURE 1;

FIGURE 3 is an end sectional view taken along the section line III-III of FIGURE 2;

FIGURE 4 is a fragmentary side elevational view, par tially in section, depicting the rear end portion of a phonograph tone arm assembly comprising a second embodiment of apparatus for balancing the outputs of a stereophonic phonograph pickup cartridge;

FIGURE 5 is an underside plan view of the apparatus shown in FIGURE 4; and

FIGURES 6 and 7 are enlarged sectional views taken along the section lines VI-VI and VII-VII, respectively, of FIGURE 5.

I have found that the teachings of my invention may be embodied in a number of constructional form. Experimental work has been conducted on apparatus for adjusting the angular relation between the stereophonic phonograph pickup cartridge and the tone arm and such apparatus is depicted in FIGURES 1-3 of the drawing. Also, the teachings of the present invention may be implemented by providing means for tilting the tone arm and the stereophonic phonograph pickup cartridge mounted therein with respect to the surface of the record disc to achieve the desired balanced outputs. This embodiment is shown in FIGURES 47 of the drawing.

Referring now to the drawing, and initially to FIG- URES 1-3 thereof, there is shown a forward end portion of a phonograph tone arm 10 which, in accordance with the usual practice, is mounted at its rear end, not shown, for free swinging movement across the surface of a record disc 11. The tone arm is generally channel shaped and hollow and mounted in the enlarged forward end portion 12 thereof, in a manner to be more fully explained, is a stereophonic phonograph pickup cartridge 14. The stereophonic pickup cartridge 14 may be of any well known construction, such as, for example, is fully disclosed in the co-pending application of Henry J. Cvetko and Maurice M. Palo, Serial No. 782,571, filed December 23, 1958, entitled stereophonic Phonograph Pickup Cartridge, which is assigned to the assignee of the present invention. cartridge 14 comprises an operatively disposed groove engaging tip 15 which is adapted to track the continuous groove in the record disc 11 and the pickup cartridge is also equipped with a second groove engaging tip 16 which may be adapted for playing different kinds of record discs.

The stereophonic phonograph pickup cartridge is rigidly but releaseably secured in a generally tubular holder 17 that is provided with a transversely extending operating arm 18.and a plurality of rearwardly pro jecting output terminals 19. The output terminals 19 are connected to suitable amplifying and speaker apparatus, not shown, by means of the twisted leads 24 extending the length of the phonograph tone arm 10. The arrangement is such that the undulations in the two .channels. in the groove of the record disc that are sensed by the operatively disposed groove engaging tip 15 are converted by the transducer elements of the pickup cartridge, not shown, into proportional electrical impulses. or outputs that may be evidenced across pairs of the output.

terminals 19.v One output is provided for each of the two channels recorded in the groove of the record disc 11. The electrical impulse or outputs are then conducted by the leads 24 to the associated amplifying and speaker apparatus whereby the same are amplified and reproduced as audible sound.

The holder 17 for the stereophonic phonograph pickup.

cartridge 14 is journaled for rotation in a generally U- shaped holder 21 whereby the pickup cartridge may be turned over conveniently upon manipulation of the lever 18 to place the groove engaging tip 16 in operatively disposed groove engaging relation as is well known in the art. This is shown in FIGURE 3 of the drawing. The U-shaped mounting bracket 21 is provided with a pair of outwardly directed and transversely spaced mounting ears 22 and 23. The above described structure is well known in the art and, in accordance with present day practice, suitable vertically extending screws are placed through the ears 22 and 23 and are securely tightened to hold the mounting bracket rigidly against suitable mounting bosses provided in the enlarged forward end portion of the tone arm. The disposition and mounting of the component parts is such that the axis 25 of the stereophonic pickup cartridge is angled downwardly with respect to the surface of the record disc 11 and the longitudinal axis of the tone arm and coincides with the vertical physical axis of symmetry 26 of the enlarged forward end portion 12 of the tone arm 10.

In accordance with the teachings of the present invention, I provide a pair of downwardly depending mounting bosses 27 and 28 on the underside of the top wall of the tone arm which are somewhat shorter than those of the prior art. The bosses 27 and 28 are transversely spaced with respect to each other and are preferably integrally molded or otherwise formed with the tone arm. The bosses have vertical bores extending therethrough which are adapted to loosely receive the ends of elongated and threaded screws 29 and 30. The headed ends 31 of the screws 29 and are received below the transversely spaced and projecting ears 22 and 23 to carry and support the mounting bracket 21, the holder 19 and the stereophonic phonograph pickup cartridge 14. The screws 29 and 30 are sufficiently long whereby their threaded ends project above the top wall of the tone The stereophonic phonograph pickup.

arm and suitable knurled and internally threaded knobs 32 and 33 are threadably received over the upwardly projecting ends of the screws. Encircling the bosses 27 and 28 are a pair of tensioned coil springs 34 and 35 whose one set of ends bear against the underside of the tone arm and whose other set of ends engage the pair of transversely spaced mounting cars 22 and 23 of the mounting bracket 21. It will thus be seen that the coil springs 34 and 35 forcibly press the cars 22 and 23 into engagement with the headed ends 31 of the screws 29 and 30.

When the phonographpickup cartridge is mounted in the manner illustrated in FIGURE 3 of the drawingthat is when equal lengths of the screws 29' and 30' are disposed below the top wall of the tone arm-the center line 25 of the stereophonic phonograph pickup cartridge is dis,- p-osed along and coincides with the axis of symmetry 26 of the enlarged forward end portion of the tone arm. However, turning of eitherv of the, knurled knobs 32 and 33 in either direction will effectively change the relative lengths of the screws disposed below the top wall of the tone arm whereby the stereophonic phonograph pickup cartridge assumes a tilted position with respect to the axis of symmetry 26 of the tone arm. Thus, the tilted or rotational position of the pickup cartridge with respect to the tone arm may be adjusted whereby its axis 25 lies along a plane 37 tilted to the right with respect to the 'axis of symmetry 26 of the tone arm or alternately along a plane 38 which is tilted to the left with respect to the axis of symmetry 'of the tone arm, for example.

As described above, the stereophonic phonograph pickup cartridge responds to a pair of signals, each of which is separately recorded on an opposite side wall of a single record groove or trace. When the outputs of the cartridge 14 are not balanced and absolutely equal for equal undulations in both of the channelslthe unbalance in the outputs of the pickup cartridge may be corrected by manipulating the knurled knobs 32 and 33 to change the relative angular disposition between theaxis 25 of the phonograph pickup cartridge and the axis of symmetry 26 of the tone arm.

This in effect changes the angle of attack of or engagemen-t between the operatively disposed groove engaging tip and the two channels in thecontinuous groove and the surface of the record disc. Thus, the operatively disposed groove engaging tip is positioned in better driven relation with one orthe other of the channels in the groove of the record disc whereby the outputs of the stereophonic phonograph pickup cartridge are equally balanced.

Unbalanced outputs may result from unequal forces be:

ingapplied to the pickup cartridge by the tone arm 10,

from structural faults in the pickup cartridge. and from in-.

herent differences in the properties of the component parts thereof. Any and all unbalance in the outputsof the stereophonic phonograph pickup cartridge may be corrected and compensatedfor by changing the angle of attack between the operatively disposed groove engaging tip and the channels of the groove in the record disc as abovev described.

A further advantage of the above described apparatus is that the pickup cartridge is resiliently mounted by the springs 34 and 35. The springs 34 and 35 are tensioned to such an extent that the ears 22 and 23 of the holder,

21 are maintained in tight engagement with the headed ends 31 of the screws 29 and 30 during normal use of the tone arm and the stereophonic phonograph pickup cartridge. However, if the pickup cartridge is subjected to an excessive vertical force, due to operator carelessness in dropping the tone arm, for example, the springs 34 and 35 will be further compressed to absorb this force. The arrangement is such that the pickup cartridge is better adapted to withstand excessive vertical forces and this is an important safety feature.

Considering now the embodiment shown in FIGURES 4-7 of the drawing, there is shown the rear end portion 40 of a tone arm which is supported for universal pivotal movement by mounting means, to be hereinafter more fully described, from a mountnig board 41. The mounting means comprising a base assembly 42 which is secured to the mounting board 41 by a nut 43 threadably received on the lower end portion of the base assembly. An inner pivot tube 43 is journaled in the base assembly 42 and it establishes a vertical axis 44 about which the tone arm may be laterally pivoted. Secured at the upper end of the pivot tube 43 is a rearwardly extending plate 45 which has a pair of downwardly depending ears 46 and 47 formed along the edges thereof. A transversely extending pivot pin 48 spans the ears 46 and 47 and establishes a horizontal axis represented by the line 49 about which the tone arm 40 may pivot vertically.

The transversely extending pivot pin 48 is snugly received in an aperture 50 in the inner side wall of the tone arm and the end of the pivot pin projects outwardly beyond this inner side wall. In the outer side wall of the tone arm 40 there is provided a vertically elongated aperture 5-1 through which projects the headed end portion of the transversely extending pivot pin 48.

To further and adjustably secure the tone arm to the transversely extending pivot pin 48 a block 53 of resilient and relatively hard material is mounted on the transversely extending pivot pin 48 intermediate the ends thereof and inwardly of the ears '46 and 47. It will be noted that the block 53 is mounted adjacent the ear 46 and the outer side wall of the tone arm and is offset a substantial distance from the vertical axis 44 defined by the pivot tube 43. The block 53 extends through an aperture 54 in the plate 45 and has a socket 55 formed in the upper surface thereof. Snap fitted in the socket 55 is a ball 56 which is mounted at the end of an elongated threaded portion 57 of adjustment knob 58. The elongated threaded portion 57 of the adjustment knob 58 is threadably received in a vertically disposed aperture 59 formed in the top wall of the tone arm 40 and the vertical axis of the adjustment knob 58 is offset laterally with respect to the vertical axis 44 defined by the pivot tube 43. It should be evident that upon proper turning movement of the adjustment knob 58, forces may be brought to bear on the transversely extending pivot pin 48 to cause an inward or outward tipping of the tone arm. Thus, if the adjustment knob 58 is rotated in one direction from the position shown in FIGURE 7 to effectively increase the length of the elongated threaded portion '57 of the adjustment knob 58 disposed between the 'block 53 and the inner surface of the top wall of the tone arm, the tone arm will be tilted with respect to the axis 49 of the transversely extending pivot pin 48 with the outer side wall of the tone arm disposed above the inner side wall thereof. A line interconnecting the side walls of the tone arm and lying along the axis '49 with the parts disposed as shown in FIGURE 7 may now extend along the line 60. Conversely, if the adjustment knob 58 is rotated in the other direction the length of the elongated threaded portion 57 of the adjustment knob 58 disposed between the block 53 and the inner surface of the top wall of the tone arm will be decreased to tilt the tone arm in the opposite direction. The above mentioned line interconnecting the side walls of the tone tan-n may then be coincident with the line 61 whereby the outer side wall of the tone arm is disposed below the inner side wall thereof. Manipulation of the adjustment knob 58 will cause tipping of the tone arm to change the angle of attack between the operatively disposed groove engaging tip of the stereophonic phonograph pickup cartridge mounted in the outer free end of the tone arm and the two channels in the groove of a record disc.

The mechanism above described comprises a second embodiment of the invention and is particularly adapted for use where the phonograph pickup cartridge is nonadjustably and rigidly secured in the free end of a tone arm. If outputs from the two halves of the stereophonic 6 phonograph pickup cartridge are not matched and are not absolutely equal, the unbalance may be corrected by appropriately manipulating adjustment knob 58 to properly tilt the tone arm.

One important aspect of the invention is the ease with which present and prior art reproducing systems may be modified to include the apparatus described above. At most, the user need only procure a new tone arm with its associated adjustment knob and block or knobs, coil springs and elongated screws.

Another important aspect of the present invention is the extreme simplicity of the apparatus. The same is adapted to be readily manufactured at a relatively loW cost whereby it may serve the mass market.

It should be apparent that I have accomplished the objects initially set forth by providing improved apparatus for balancing the outputs of a stereophonic phonograph pickup cartridge. Although two representative embodiments of such apparatus have been disclosed, many changes may be made therein without departing from the clear teachings of the invention. Accordingly, reference should be had to the following appended claims in determining the true scope and intent of the invention.

I claim:

1. Apparatus for balancing the outputs of a stereophonic phonograph pickup cartridge of the type adapted for use in reproducing two channels of sound recorded on opposite sides of a single groove and having a stylus, comprising means to support said cartridge to maintain a predetermined transverse angular disposition between said stylus and record groove, and means to move the cartridge with respect to said support and thus to adjust the transverse angular disposition of said cartridge in a plane extending normally with respect to the center line of said groove.

2. Apparatus according to claim 1 characterized in that said stereophonic phonograph pickup cartridge is mounted in a tone arm, and said means to adjust comprising means to change the angular disposition of said cartridge in a plane extending generally normally with respect to the center line of said tone arm.

3. Apparatus according to claim 1 characterized in that said stereophonic phonograph pickup cartridge is mounted in a tone arm, means mounting said tone arm for pivotal movement at a point spaced from said stereophonic phonograph pickup cartridge, and said means mounting comprising means to adjustably tilt said tone arm and said stereophonic pickup cartridge en masse in a plane extending normally with respect to said center line of said groove.

4. Apparatus according to claim 1 characterized in that said stereophonic phonograph pickup cartridge is mounted in a tone arm, means mounting said tone arm for pivotal movement from a support at a point spaced from said stereophonic phonograph pickup cartridge, and said means to adjust comprising means to tilt said tone arm and said stereophonic phonograph pickup cartridge en masse with respect to said support.

5. Apparatus for balancing the outputs of a stereophonic phonograph pickup cartridge of the type adapted for use in reproducing channels of recorded intelligence from channel means, said stereophonic phonograph pickup cartridge comprising channel sensing means, and means to adjust and retain the angular disposition of said channel sensing means in a plane extending normally with respect to said channel means to change the angle of attack between said channel sensing means and said channel means, a tone arm, means mounting said stereophonic phonograph pickup cartridge in said tone arm, and said means mounting comprising said means to adjust whereby the angular disposition of said stereophonic phonograph pickup cartridge in a plane extending generally normally with respect to the center line of said tone arm may be adjustably varied.

6. Apparatus. according to claim.5 characterized inthat said means mounting. comprises a. bracket having a. pair of outwardlyextending and transversely spaced, ears, said stereophonic pickup cartridge being mounted in said. bracket, a pair of headed elongated screws extending vertically through said ears whereby said ears rest on the headed portions .of said: screws, said tone arm, having a top wall,- said-screws projecting above said top *wall, inter: nally threaded knobs. threadably received on the ends of said screws projecting above said top wall of said tone arm, and a pair of coil springs encircling said. screws and extendingbetween saidears and the underside of said top. wall.

7. Apparatus according to claim characterizedin that said tone arm further comprises a top wall, a pair. of vertically extending members, said vertically extending members being connected with and carrying said stereo phonic phonograph pickup cartridge on opposite sides of the center line thereof, said vertically extending members projecting through said top wall, and means to .adjustably and individually vary the lengths of said vertically extending members disposed below said top wall of said tone arm.

8. Means for mounting a phonograph pickup cartridge for. transverse tilting movement with respect to the center line of a tone arm having a top Wall comprising a pair of vertically extending members, said vertically extending members being connected with andcarrying said phonograph pickup cartridge on. opposite. sides of and outwardly of; the center line thereof,.said vertically extending members projecting through said top Wall of saidtone arm, and means to adjustably and individually vary the lengths of said vertically extending members disposed below said top wall of said tone arm.

9. Apparatus according to claim 8 characterized in that said mounting means further comprises a pair of ears projecting outwardlyrfrom said phonograph pickup cartridge, said vertically extending members having headed endportions, said vertically extending member extending through saidears whereby saidears rest on said headed portions, and spring means biasing said ears into engagem'entwith'said headed portions of said vertically extending members.

10. Apparatus for balancing the outputs of a stereo phonic pickup cartridge .of the typeadapted for use in reproducing channels of recorded intelligence, said stereophonic pickup cartridge comprising channel sensing means, a tone arm, means mounting said stereophonic pickup cartridge in said tonearm, means. mounting said tone arm for pivotal movement from a support at a point spaced fromsaid stereophonic pickup cartridge, and said means mounting comprising means to adjustably. tilt said tone armxabout its longitudinal axis and to retain said tone arm in tilted position whereby the angle of attack between said, channel sensing means and said channels may be adjustably varied.

11. Apparatus for balancing the outputs of a stereophonic pickup cartridge of the type adapted for use in reproducing. channels of recorded intelligence, said stereophonic pickup cartridge comprising channel sensing means, a tonearm,- means mounting said stereophonic pickup cartridge in said .tone arnnmeans mounting said tone arm forqpivotalmovement from a support at a point spaced:

fronrsaid stereophonicpickup cartridge, and said means mounting comprising meansdtoadjustably tilt said tone, armabout its longitudinalaxis and .to retainsa-id tone arm: in tilted :positionzwhereby the, angle of attack between said; channel sensing means and, said channels maybe adjust-.

ably varied; said tone ,arm comprising-a top walLand-a pair of transversely spacedvertically extending side Walla. a transversely extending pivot pin spanning said side walls, means supporting said pivotpin from said support, saidpivot pin being rigidly attached. to one of saidside walls,

an elongated aperturev in the other of said side walls through'which said pin loosely. extends, and adjustment means ofisetlaterally:fromsaid longitudinal axis of said tonearm adjacent said other. side wall for adjustably varying the distance between said top wall and said transversely extending pivot pin.

12. Apparatus according to claim 11 characterized in that said adjustment meanscomprises a resilient and relatively hard block mounted on said tone arm adjacent said other side wall of saidtone arm,.said block having a socket in theupper face thereof, an adjustment knob having an elongated threaded portion, a threaded aperture in said-top wall, said-elongated threadedportion'being pin spanning said-side walls, said pivot pin being attachedto one of said side walls, and adjustable means adjacent the other of said sidewalls for adjustablyvarying the distancev between said top wall and said pivot pin to allow adjustable tilting of said tone arm about its longitudinal axis.

14. Apparatus according to claim 13 characterized in that said adjustable means comprises a block mounted on said pivot pin. adjacent said other side wall, a vertically extending member connected with said block and projecting through said top wall,'and means to adjustably vary the length of said vertically extendingamember disposed between said top wall and said-block.

15. Apparatus according to claim l4 characterized in that-said block has a socket ,formed therein, said block being,formedfromresilient and relatively hard material, said vertically extending member-having a ball. at one .end thereof, and saidball being seatedin said socket.

References Cited in the file of this patent UNITED STATES PATENTS 2,568,671 Vistain Sept. 13, 1951 2,582,948- Cairnes Jan. 22, 1952 2,776,144 Nichols Jan. 1, 1957 3,028,161 Siebert Apr. 3, 1962 

11. APPARTUS FOR BALANCING TE OUTPUTS OF A STEREOPHONIC PICKUP CARTRIDGE OF THE TYPE ADAPTED FOR USE IN REPRODUCING CHANNELS OF RECORDED INTELLIGENCE, SAID STEREOPHONIC PICKUP CARTRIDGE COMPRISING CHANNEL SENSING MEANS, A TONE ARM, MEANS MOUNTING SAID STEREOPHONIC PICKUP CARTRIDGE IN SAID TONE ARM, MEANS MOUNTED SAID TONE ARM FOR PIVOTAL MOVEMENT FROM A SUPPORT AT A POINT SPACED FROM SAID STEREOPHONIC PICKUP CARTRIDGE, AND SAID MEANS MOUNTING COMPRISING MEANS TO ADJUSTABLY TILT SAID TONE ARM ABOUT ITS LONGITUDINAL AXIS AND TO RETAIN SAID TONE ARM IN TILTED POSITION WHEREBY THE ANGLE OF ATTACK BETWEEN SAID CHANNEL SENSING MEANS AND SAID CHANNELS MAY BE ADJUSTABLY VARIED, SAID TONE ARM COMPRISING A TOP WALL AND A PAIR OF TRANSVERSELY SPACED VERTICALLY EXTENDING SIDE WALLS, A TRANSVERSELY EXTENDING PIVOT PIN SPANNING SAID SUPPORT, SAID MEANS SUPPORTING SAID PIVOT PIN FROM SAID SUPPORT, SAID PIVOT PIN BEING RIGIDLY ATTACHED TO ONE OF SAID WALLS, AN ELONGATED APERTURE IN THE OTHER OF SAID SIDE WALLS THROUGH WHICH SAID PIN LOOSELY EXTENDS, AND ADJUSTMENT MEANS OFFSET LATERALLY FROM SAID LONGITUDINAL AXIS OF SAID TONE ARM ADJACENT SAID OTHER SIDE WALL FOR ADJUSTABLY VARYING THE DISTANCE BETWEEN SAID TOP WALL AND SAID TRANSVERSELY EXTENDING PIVOT PIN. 